The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Typically, large motors, generators and transformers (i.e. large electric machines) remove internal heat in the following fashion. Air is ingested at one or both ends of the rotor. The air can be ingested solely by the suction created by the pumping action of the rotor or assisted by external blowers. Part of the ingested cooling air blows directly over the stator coil end turns. It should be noted that some electric machines have the air that has passed through the rotor and stator and is slightly heated blow over the exposed coil end turns before exiting (as is the case with single end ventilated machines).
The ingested air travels axially down the rotor in the space between the shaft spider bars and is pumped through the rotor radial vents by the rotor bars. The air is then pumped through the air gap (the radial space between the stator and the rotor) and into the stator radial air vents. The air enters the stator radial air vent and blows over the exposed coil end head (i.e. the coil section in the vent section and not the core section where the coil is surrounded by the stator core iron). The air then blows through the rest of the radial vent until it is exhausted at the outside diameter of the stator core. The radial vent sections alternate with the core pack sections. A typical core section is 1.75″ long and a typical radial vent sections is 0.5″ wide. However, the sizes and proportions of the core packs and air vent sections vary significantly from one design to the next. In addition, some designs have the stator and rotor vent packs aligned with each other, while in other designs they are offset.
The above description does not include the airflow through the frame, bearing brackets, air enclosures, etc., as these details are machine specific and commonly known to one skilled in the art.
The power density of such electric machines is thermally limited by the stator coil temperature. In particular, the stator coil temperature is limited by the maximum allowable temperature of the electrical insulation system (i.e. the insulation that surrounds the coil). It should also be noted that approximately 50% of the total motor losses (which manifest themselves as heat) are generated within the stator coils. The heat that is generated within the coil has three parallel paths by which it can be shed: 1) convection in the small section of exposed coil (i.e. the part of the coil that is in the vent packet area) directly cooled by the air flow; 2) conduction in the larger section of stator coil that is surrounded by the stator core iron (i.e. the part of the coil that is in the core packet section). This area is cooled by conducting heat from the coil to the core iron, and the again by conduction as the heat travels axially in the core pack section until it gets to the outermost lamination in the core pack area. This outermost stator iron lamination has the heat is removed via convection by the cooling air as it travels in the radial air vent; and 3) convection in the exposed coil end turns directly cooled by the air flow.
The stator coil temperature is a balance of how much heat is generated in the coil and how effectively this heat is rejected through the three parallel paths as described above. This maximum coil temperature limits the maximum power that a particular machine can produce.
As described above, approximately half of the heat is generated in the coil. A large proportion of the coil is surrounded by the core iron, and thus not effectively cooled. For instance, if the air vent is 0.5″ wide and the stator core pack length is 1.75″, then only 22% of the coil length is directly exposed to the cooling air. The thermal resistance is very high between the coil in the core pack area and the face of the radial vent (this is also the outermost lamination of the core pack) where it is convected to the air stream. This high thermal resistance path is the only heat transfer path for the larger stator coil area where most of the heat generated in the coil is generated. Details for this long, torturous heat transfer path resulting in the high thermal resistance is as follows.
Within a specific core pack section, heat is conducted through the center portion of the stator coil to the outer edges of the coil. This heat is then conducted from the outer surface of the stator coil through the electrical insulation. The coil insulation has a high thermal resistance, but it is very thin (typically 0.030″ per side for medium voltage (4000 Volt) electric machine, but the actual thickness varies significantly from one design to the next). The heat is then conducted from the insulation to the stator slot edges in the stator core. The edges are formed from the individual laminations. All the laminations in the core pack are make up a core pack area.
Heat is conducted radially and axially in the stator core from the slot area to the surface of the radial air vent. The stator core is not a solid block of steel, but consists of many very thin steel plates (i.e. the stator laminations). These laminations have insulation on the surfaces which further raises the thermal resistance and inhibits heat transfer in the axial direction.
Typical stator laminations are 0.018″ thick. Lamination thickness varies significantly from design to the next. This example of an electrical machine with a 1.75″ core packet length would consist of 95 individual laminations. The net impact of high thermal resistance is that it takes a greater temperature differential to move a given amount of heat. Conversely, the amount of heat that is transferred is limited by the temperature rise between the where the heat is generated (the stator coil) and where it is rejected (the air flowing through the radial air vent). Electrical machines are designed to balance the amount of heat that can be removed while staying below the maximum temperature limit at the coil. Very often, temperature sensing devices (such as resistance temperature devices (RTDs)) are placed directly in the stator slot to measure the temperature in the coil at the slot to assure that the maximum temperature limit is not exceeded. The stator coil is shown in FIG. 4.
Finally, the heat is convected to the air flowing through the radial air vents. There are two paths where the flowing air absorbs the heat: the air flowing directly over the short exposed section of stator coil and the air which is flowing through the rest of the air vent absorbing the heat that has been conducted through the stator core.